Method for the production of neutral sulfite pulp



United States Patent 3,078,208 METHOD FOR THE PRODUCTION OF NEUTRAL SULFITE PULP Karl Nicolaus Cederquist, Falun, and Karl Esbjiirn Eftring, Kvarnsveden, Sweden, assignors to Store Kopparbergs Bergslags Aktieboiag, Falun, Sweden, a joint-stock company limited of Sweden No Drawing. Filed Sept. 10, 1959, Ser. No. 839,071 Claims priority, application Sweden Sept. 11, 1958 2 Claims. (Cl. 162--68) The present invention relates to the production of sulfite pulp. A principal object of the invention is to provide an improved method for the production of semichemical neutral sulfite pulp.

Neutral sulfite pulp usually is produced in yields of 65 to 90% by digesting wood with a disodium sulfite solution buffered with soda at temperatures within the range of 160 to 180 C. and by subsequent mechanical defibra-.

tion of the digested pulp.

In heating wood within the above mentioned temperature range, there are formed organic acids causing a brown discoloration of the wood substance. In order in some degree to counteract this drawback, there is added to the sodium sulfite solution sodium carbonate or bicarbonate so that the buffering capacity of the solution is increased. The soda addition usually amounts to about A of the sulfite charged. Due to the improved buffering capacity organic acids formed are bound to a greater extent and the discoloration of the wood becomes less pronounced. A pure sodium sulfite solution has a pH of about 9.5 and is void of buffering capacity. The digesting liquor generally is prepared by dissolving therein sodium sulfite and sodium carbonate or bicarbonate but in some cases it is prepared also by introducing S0 into a soda solution, until the desired proportion of sodium sulfite and sodium carbonate is obtained.

However, experiments have revealed that by an impregnation of the wood chips carefully accomplished it is not necessary for the sulfite solution to have any appreciable buffering capacity within the pH range of 9.5 to 7.5. Furthermore, it was found that the content of carbonic acid, free or combined, of the sulfite solution is positively deleterious to the digestion process, since soda or bicarbonate will decompose Within the wood chips with the formation of gaseous carbonic acid which displaces part of the cooking liquor, and the neutralization of the acids formed is rendered more difficult, whereby the risk of the brown-coloration of the wood chips is increased. It has also been found that in using solutions buffered with soda or bicarbonate the impregnation process is rendered more difiicult. Gaseous carbonic acid also reduces the partial steam pressure, for which reason an adequate quantity of steam must constantly be tapped to enable the maintenance of the desired reaction temperature.

It has also been found that in carefully effecting the impregnation of the wood the brightness of the pulp will be increased, if the pH of the impregnation solution is lowered within the range of 9.5 to 6, however provided of course the digestion is effected under similar conditions with respect to temperature and so forth.

Contrary to prior opinion, it has proved to be extremely important that the wood chips, before being heated above the range of 110 to 120 C., be Well impregnated with cooking liquor, if a discoloration and undue breakdown of the wood substance is to be obviated. If the wood chips are not completely soaked through by the cooking liquor, in heating the wood chips within the above mentioned temperature range, there occurs an incipient discoloration of the pulp which manifests itself still more at the temperature range of 140 to 180 C., commonly used in practice.

A steaming of the wood chips greatly facilitates the ice impregnation and the duration of the steaming is dependent upon the type of the wood, percentage dry weight, the size of the wood chips, etc. For chips made of various types of birchwood it has, for instance, been necessary to vary the steaming period between 5 and 10 minutes to obtain a satisfactory impregnation.

In a well performed steaming operation normal chips of birchwood, dependent on the moisture content after the steaming operation, must be in contact with the cooking liquor for /2 to 2 hours, if the impregnation is to be good enough so that there is imparted a maximum of brightness to the pulp. With respect to the critical temperature range mentioned hereinabove, it is also very important that during the impregnation period the temperatures are kept below 110 C., suitably at about 100 C. or lower.

It has been heretofore observed that in using a sodium sulfite solution which is not buffered the color of the pulp obtains a greater brightness than that with a buffer, but the brightness heretofore obtained is very low, 52% GB. in comparison with the result attained by the instant invention, and this must be ascribed to a too high temperature during impregnation 120 C.).

Tests have shown that by carefully effecting the impregnation it is possible to attain a brightness for birch-. wood of 62 to 65% in the yield range of to if in the impregnation there has been taken due regard of the maximum allowable temperature of to C. and the necessary time of 30 to 120 minutes.

The duration of the impregnation is at least as important as the temperature to obtain a perfectly satisfactory impregnation. We have found that, to attain a satisfactory result with respect to impregnation, it is necessary that well steamed wood chips be contacted with the impregnation solution for at least 30 minutes and that the impregnation time frequently must be extended to 60 to 120 minutes.

It has also been previously stated that the brightness of pulp is greatly dependent on impregnation, but the impregnation time previously used, 15 minutes at 100 C., is too short to give a satisfactory brightness.

If the impregnation is carried out in such a manner,

that, after steaming, the wood chips are compressed and allowed to expand directly in the cooking liquor, for instance according to the so-called Asplund method, the impregnation time may be substantially shortened. Tests have shown however, that the impregnation time cannot be made shorter than 3 to 10 minutes, if for instance birch wood pulp is to get a brightness of about 60 to 65 GE.

It has been stated that an impregnation liquor prepared by introducing $0 into a sodium hydroxide solution to a pH within the range of 9.5 to 6, preferably between about 7.5 to 6.5, is more suitable than any other sulfite solution for the production of neutral sulfite pulp of high brightness at a yield of between 80 and 90%. In that case the cooking must be carried out in such a manner that after' completed cooking the pH value resides between 4.5 and 6.5 and the cooking temperature between and C., which for the highest temperature corresponds to a cooking time of 15 to 20 minutes and for the lowest temperature to a cooking time of 60 to 70 minutes. It should be 3 strength at a high yield of 80 to 90% and of a brightness Quits su fici nt fo h p od ctio o wsp i and m azine paper.

With respect to the above considerations, the present invention has as an object the provision of a method for the production of semichemical neutral sultite pulp, said method being characterized by the steps of carefully impregnating wood chips, which have been steamed, with a solution of sodium sulfite and bisulfite having a pH of about 9.5 to 6 and void of free or combined carbonic acid, removing the excess of impregnation solution Without any free liquor, digesting the pulp at a temperature of 165 to 175 C and subsequently defibrating the pulp.

According to one embodiment of the invention, the wood chips are steamed, compressed and subsequently expanded in the impregnation liquor after having been maintained therein for at least 3 minutes, and preferably more than 10 minutes.

Furthermore, it has been found that a further increase of the brightness may be attained if subsequent to completed cooking and defibration, the pulp suspension is freed from waste liquor by compressing to increase the dry content Without adding any dilution water. Since according to the method described the digestion is effected without any free liquor, it may be necessary after the coarse defibration to add a minor quantity of liquor to facilitate the conveyance of the pulp. Then preferably undiluted waste liquor from the press should be used. The compression is suitably effected in continuous presses such as screw presses or disc presses of a type common in the wood pu ind r Even through a compression to a dry content of 30% prior to the dilution with Water, there is obtained an increase of the brightness of about 2 GE. units as compared with a Washing-out in a diffusor. In compressing to a dry content of 40% the increase of the brightness has amounted to about 3 GE. units. At a dry content of 50% the brightness has increased by about 1 GE. unit more.

Subsequent to the compression the pulp is diluted with water to a concentration of 3 to 4%, refined, diluted, washed and formed to a paper web. The determination of the brightness was performed with the paper thus obtained.

It has been found that the etfect obtained is due to the fact that in diluting or washing the pulp suspension subsequent to the completed cooking operation, small quantities of granular organic substance are precipitated and adsorbed by the fibres, through which the brightness of the pulp is decreased. This fact has been recognized in the production of high yield pulp of hardwood such as birch and poplar. High yield s ulfite pulp produced from softwood (spruce) has the same tendency, although the increase of the brightness is considerably lower.

Through the invention it is furthermore rendered possible at a relatively low charge of chemicals amounting to maximum 42 kilos of Na O and 45 kilos of S pro metric ton dry weight of the wood to obtain a brightness not heretofore attained.

In carrying out the impregnation according to the invention it is attained that all parts of a wood chip are impregnated with a predetermined quantity of chemicals, without there being present at any part of the wood chip an undue excess, resulting in a maximum brightness and Table II Irnpregn. 1 Take-up of pH Brighthr., p NazO, chem, gJkg. after Yield, uess, Batch percookperpercent ing cent cent Before After N320 S0:

Table III Impregn. 1 Take-up of pH Bright hr., pH NaeO, chem., gJkg. alter Yicld, ness, Batch percookperper cent iug cent cent Before After NarO SO:

Table IV Impregn. 1 Take-up of pH Bright hr., pII N220, chem, git-2g. alter Yield, ness, Batch percookperpercent ing cent; cent Before After NazO S02 Having now particularly described and ascertained the nature of our said invention and which manner the same is to be performed, we declare, that what we claim, is:

1. A method comprising preparing semichemical neutral snlfite pulp in a yield of at least about and to a brightness of at least 60 GE. by the steps of making an tmbutfered solution consisting of sodium sulfite and bisulfite and in a pH range of 9.5 to 6, thoroughly steaming Wood chips, subsequently impregnating said chips with said solution at a temperature no greater than about C., removing all free solution to provide a dry content of at least 30%, digesting the chips in steam in the absence of free solution at a temperature of from to C., and defibrating the resulting pulp.

2. A method as claimed in claim 1 wherein the chips are impregnated by being mechanically compressed and then expanded in said solution in which the chips are maintained for at least three minutes, all free solution being subsequently removed.

References Cited in the file of this patent UNITED STATES PATENTS 215,728 Farrell May 27, 1879 2,640,774 Ross June 2, 1953 2,858,213 Durant Oct. 28, 1958 2,862,813 Birdseye Dec. 2, 1958 2,975,096 Ginaven Mar. 14, 1961 OTHER REFERENCES Casey: Pulp and Paper, vol. I, published by Interscience Publ, New York, N .Y., 1952, pp. 206-211. 

1. A METHOD COMPRISING PREPARING SEMICHEMICAL NEUTRAL SULFITE PULP IN A YIELD OF AT LEAST ABOUT 80% AND TO A BRIGHTNESS OF AT LEAST 60 G.E. BY THE STEPS OF MAKING AN UNBUFFERED SOLUTION CONSISTING OF SODIUM SULFITE AND BISULFITE AND IN A PH RANGE OF 9.5 TO 6, THOROUGHLY STEAMING WOOD CHIPS, SUBSEQUENTLY IMPREGNATING SAID CHIPS WITH SAID SOLUTION AT A TEMPERATURE NO GREATER THAN ABOUT 110* C., REMOVING ALL FREE SOLUTION TO PROVIDE A DRY CONTENT OF AT LEAST 30%, DIGESTING THE CHIPS IN STEAM IN THE ABSENCE OF FREE SOLUTION AT A TEMPERATURE OF FROM 165 TO 175*C., AND DEFIBRATING THE RESULTING PULP. 